As clinical trials of gene Therapy have begun, Immune reactions to therapeutic gene products have been encountered. Our preliminary studies in a canine model of Hurler's disease (Mucopolysaccharidosis Type I [MPS I], resulting from deficiency of the lysosomal enzyme alpha- L-Iduronidase (IDUA)), have demonstrated immunologic reactions to normal IDUA protein, either given as recombinant enzyme or via transduced autologous bone marrow or myoblasts. The immune responses against the transgene products interfere with enzyme distribution and limit the persistence of gene-expressing cells. With the advent of a system for producing large quantities of recombinant IDUA, it is now possible to study antigenic tolerance induction using the recombinant protein antigen. Alternatively, the cytoablative conditioning given prior to bone marrow transplantation (BMT) is potently immunosuppressive and immune reconstitution post-BMT results in tolerance to exogenous antigens present in the engrafted bone marrow. Our central hypothesis is that induction of tolerance to IDUA by the introduction of the normal IDUA cDNA via bone marrow stem calls following cytoablative conditioning or by pre-treatment with intra-thymic injections of recombinant IDUA protein will prevent the immune response and allow persistence of cells expressing IDUA enzyme. To test this hypothesis, studies will be done using murine and canine MPS I models. We will determine the role of immune responses against transplanted murine bone marrow cells transduced by exogenous human IDUA cDNA on the persistence of cells expressing the foreign gene product. The effects of immune modulation, primarily pre-transplant total body irradiation, will be determined. In the canine MPS I model, we will evaluate intra-thymic injection of recombinant IDUA as a means to induce antigen-specific tolerance assessed by intravenous injection of recombinant IDUA protein and transplantation of autologous myoblasts or bone marrow cells transduced with the IDUA cDNA. Successful tolerance induction in murine and canine models of MPS I will be an Important step toward application of gene therapy in human patients.